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IBM 700/7000 series

The IBM 700/7000 series was a series of compatable 36 bit scientific computer systems made by IBM through the 1950s and early 1960s. They were made obsolete by the introduction of the System 360.

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Instruction Format

The basic instruction format was a 3-bit prefix, 15-bit decrement, 3-bit tag, and 15-bit address. The prefix field specified the class of instruction. The decrement field often contained an immediate operand to modify the results of the operation, or was used to further define the instruction type. The three bits of the tag specified three (seven in the 7094) index registers[?], the contents of which were subtracted from the address to produce an effective address. The address field either contained an address or an immediate operand.


  • AC - 38 Bit Accumulator
  • MQ - 36 Bit Multiplier-Quotient
  • XR - 15 Bit Index Registers (three or seven)
  • SI - 36 Bit Sense Indicator

The Accumulator (and Multiplier-Quotient) registers operated in signed magnitude[?] format.

The Index registers operated using two's complement format and when used to modify an instruction address were subtracted from the address in the instruction. On machines with three index registers, if the tag had 2 or 3 bits set (i.e. selected multiple registers) then their values were ORed together before being subtracted. The IBM 7094, with seven index registers had a "compatibility" mode to permit programs from earlier machines that used this trick to continue to be used.

The Sense Indicators permitted interaction with the operator via panel switches and lights.

IBM 700 series, vacuum tubes, 1950s

IBM 7000 series, transistors, 1960s

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